When I compare these to the newer Studio K240s, these win by a long shot. Not only in neutrality and detail, but in build design too.

The Studios have a big mid bass hump that makes everything sound boomy. But the Monitor version has a very balanced frequency response that sounds almost perfect. To me, the treble is very good, but I dislike the spike most AKGs have in the 8-9 kHz range, these included. Although not as apparent with classical music (which these are great for btw), rock and pop and singing definitely is sibilant after a while. That being said, the treble adds to the detail retrieval, which may be why these are meant for studio monitoring use. I also LOVE the mids on these, and they are the phones strong point. Mids are better than the HD 600s IMO. That being said, the soundstage is kind of small compared to many headphones, but still ok.

I also have noticed the bass could be a little more, as it rolls off quite a bit in the sub bass.

EDIT: 6/3/14

I have tested these with both the ODAC+O2 combo and my vintage Nakamichi TA-2A + CDP-2A. Sounds amazing with both of course, but I prefer the Nakamichi as it powers it better and gives a little extra bass boost and it also smooths out the treble a bit. It's also meant to be driven from those kind of sources. With the ODAC+O2, its a little sibilant and bass could be a little more, but this is because of the neutrality the O2 brings. This headphone would do quite well with warm dacs and amps.

Cons: shallow earpads, slight lack of sparkle in treble, bass could be tighter, 600 ohms is hard to drive

Background:

I was looking for a studio monitor headphone for under a hundred bucks, I was going to buy the AKG K240 Studio at first but saw these "Monitors" on eBay for much cheaper, for 59 bucks I didn't hesitate to buy them although I was nervous about the 600 ohms impedance. They arrived in perfect condition with the elastics still having lots resistance, and the ear pads without any cracks, however the golden labels at the sides had fallen off (common issue). I hooked them up to my HK 3490 receiver which were being fed Apple lossless files from my computer through my uDac 2. After three minutes I was ready to return them, everything was muffled, mids, treble, bass, absolutely terrible. I was curious, and took off the earpads and the filter, what I saw shocked me. Literally half an inch of this strange dusty, black residue gunk was everywhere, it covered the six circles around the driver and was also on top of the actual driver, I think this was the dried up residue of the adhesive they used 20 something years ago to attach the cloth to the plastic. After giving both sides a thorough cleaning and dusting out the filters, they had transformed into something else.

Bass- It's there, reaches quite low but isn't too emphasized and can be muddy at times, but not terrible at all. I listen to lots of bass driven music (rap, electronic) though I wouldn't call myself a basshead and I'm quite satisfied with the bass these put out.

Mids- What these headphones excel at, a bit forward but never fatiguing, very clear and detailed, upper mids could be better.

Treble: The reason why these headphones are never fatiguing is because of the rolled of treble. The lack of sparkle in the treble gives female voices a sometimes "dark" sound, I myself don't like a treble assault on my ears with headphones so I don't really mind this as it lets me listen to them for a long time but for you that want that pronounced treble, nothing a bit of EQ can't fix.

Soundstage: Excellent, I can distinguish locations of instruments in live performances, and also where people are firing at me from when playing Call of Duty

Comfort- I love the self adjusting headbands on this, not a lot of clamp, however these won't come off your easily, I have a slightly large head and these fit me well, I often forget I'm wearing these. I just wish the earpads were deeper and had more cushioning, my ears sometimes press against the drivers.

Build Quality: Excellent, headband is solid, plastic used is sturdy, gold accents give it a sense of quality, leather is also soft on the ear pads, besides if these weren't well built how could they have lasted 20 something years?

Aesthetics: One of the greatest looking headphones ever made.

Design- Are you kidding me? The K240 series is one of the most famous and successful headphone series' of all time spanning five generations: Sextett, Monitor, DF, Studio and MKII.

Bang for the buck- $60 bucks for this type of sound quality is unheard of, even if you pay 90 bucks these are still a great deal.

Overall: A solid choice for a used, cheap monitor. After 30+ years, the K240 is still a studio standard.

Got a deal on some of these 600 ohm out of stock beauties in mint condition and am about 3 hours into testing them. So far I am pleased with the quality and fit. Got mine hooked up to a Musical Fidelity V-can so there's no worries about power. When i first got them I hooked them up to my iphone and could barely hear them, even with the volume all the way up. I then realized how much more is needed for these and why they took the next generation k240's down to around 55 ohm. Once i burn them in I'm sure they will be a good casual listening can. Anyway, i couldn't pass up a good deal.

Why 600 ohm? Well I'm sure there's a reason to get them if you find some. I finally got my V-Can to near 11 o'clock, where it's normally at 9 o'clock or less with my AKG K701s. I think there's an advantage to being able to turn them up like hearing more of those imperfections when listening to studio mixes, hopefully not just for a future deafness. So far they are nice and nothing stands out in the lossless files I listen to, though I do like to give them a little EQing on iTunes which they seem to benefit from. Though they are not extremely bass heavy at any setting, I feel like I'm getting more of what was expected from those studio recordings.

Sweat is a small factor during long listening sessions with these. Also, can't imagine why they use a 3.5 mm plug, though my V-Can has a 3.5 mm output so it has worked out very well. I want to a-b them with my other headphones, but at the volume levels these need I will have to just listen for some time with them and then with the others I have in order to hear what I may be missing. I'm sure I'll always love my Yamaha YH-100 orthos the best of all my cans, and I don't really need these except as a throw around headphone, but we'll just have to wait and see how they burn in.

Found these classic headphones at my local thrift store for 20$! I just had to pick them up immediately when I saw them. Lucky I was, they were in perfect working order, and also cosmetically in a good shape.

The sound?

When I first heard them, I was really surprised. Since they were "monitor" headphones I expected them to sound ear piercing, with little or no bass, but wrong I was. The AKG's have a natural warm sound with solid bass and plenty of detail.

So let's compare them against the 400$ Sennheiser HD600's, and see how they hold out?

Bass:

The AKG's have slightly more oomph than the Sennheisers, but the nature of the bass is very similar; it has a nice mid bass impact, but is never overpowering. However I find the Sennheiser's bass to be tighter, while the AKG's sound slightly "bloated" at times. Both have a bass rolloff at around 50hz.

Mids:

I find the Sennheiser's have slightly more "tight" mids, in a way they sound sharper, more accurate. Sennheiser also has the edge on instrument separation maybe due to this. The K240M sound slightly compressed in comparison.

Highs:

I found the highs in the HD600's to be somewhat cleaner, more "airy" and sparkly, than the AKG's. AKG's top end sounds slightly "grainy" in comparison.

Overall:

When compared with the HD600's, the K240M's don't sound bad at all, which is quite impressive considering the HD600's cost 4 times more.

This is just a quick review, I will edit this later. Now off to sleep!

They are very comfortable and also smaller and lighter than the closed-back circumaural AKGs. Being 600Ω cans, they are somewhat dependent on the source: My old JVC CD player drives them well, so do the powerful headphone amps of older mixing boards. With less powerful outputs, e.g. digital pianos, they work ok, with portable equipment not so much.

Their sound meets my taste for general listening very well. Overall they are balanced and pleasant with reasonable attention to detail. Bass isn't as recessed as e.g. on the K271 - for listening that's a good idea IMO. Also, they lead to relatively little ear fatigue, although the don't match the K271 in that respect.

I was surprised to learn that although it is semi-open, the K240 Monitor is very dependent on the condition of its ear pads. Flat pads impair sound quality and comfort, as the baffles tend to touch the ear. Also, the new velours pads from AKG pretty much destroy their sound - so it's a good idea to stick to the leather pads, they're cheaper anyway.

As it seems to be a general consensus that those are great phones, it's hard to make a real bargain unless you're very patient on eBay. Also, it makes sense to replace elastics, ear pads and the foam disks on the baffle right away - it really makes a difference in comfort and sound.

Anyway, considering that I paid about €60 for my unit, it is a fair price for cans that serve as workhorses and for relaxed listening as well. As I said, of the three AKGs I have, those are my all-time favourite.